Abnormal sense of intention preceding voluntary movement in patients with psychogenic tremor.

Psychogenic tremor is the commonest psychogenic movement disorder, yet little is known of its pathophysiology. Given the presence of movements that appear from their physiological properties to be voluntarily produced, and yet are not experienced as such by the patients, we hypothesised that patients might have an abnormal conscious experience of volition with regard to self-generated movement. Nine patients with psychogenic tremor were asked to judge the timing of a self-paced button press relative to a clock displayed on a computer screen. In separate trials they were asked to judge the timing of their internal feeling of intention to move. These results were compared to those of healthy control participants. Patients with psychogenic tremor judged their feeling of intention to move significantly later compared to control participants. As a result, the interval between the perceived time of intention and the perceived time of action, which was highly significant in the control participants, was numerically smaller and non-significant in the patients. This study provides novel data that the sense of volition prior to movement is impaired in patients with psychogenic tremor. This fits with a pathophysiological explanation for this disorder based on an impairment of neural mechanisms that generate the conscious experience of action: actions that are voluntary in terms of their physiological origin might be experienced as involuntary.